1. Field of the Invention
The present invention relates to an exposure system for printing a pattern on a substrate such as a semiconductor wafer and a plate for a liquid crystal, and more specifically to a mark position determining apparatus used in such an exposure system for the purpose of attaining alignment between the substrate and for example a projected pattern of a reticle.
2. Description of Related Art
In this type of exposure system, before a pattern is actually printed, a preparation operation is performed for each of substrates in order to precisely position a substrate in relation to the exposure system. Conventionally, this preparation operation includes the steps of irradiating a detecting light (laser beam) through a condenser lens to an alignment mark on a wafer, and guiding a reflected light to a detecting optical system, where the reflected light is detected by an image sensor and converted into an image signal. A waveform of the image signal obtained is compared to a predetermined slice level, in order to determine a position of the alignment mark. In this case, a method for introducing the reflected light to the detecting optical system is divided into a bright-field alignment system and a dark-field alignment system.
This position determining apparatus is called a "stepper" in the field of a semiconductor device manufacturing. An overview of one recent stepper is made in "An Advanced H-Line Stepper" by Jere D Buckley, Solid State Technology, January 1987, pages 87-93. The content of this paper is incorporated therein with reference thereto.
In the dark-field alignment system, for example, a spatial filter is located before the image sensor so as to block a light reflected from a center flat surface of the alignment mark but to pass scattered lights reflected from a pair of side surfaces of the alignment mark. In other words, the spatial filter is composed of a shield plate having small windows which allow passage of only the scattered lights from the side surfaces of the alignment mark.
However, if the alignment mark has a different shape, the direction of the scattered light correspondingly changes. In some cases, the scattered light reflected from the side surfaces of the alignment mark is almost or completely blocked by the spatial filter, with the result that a sufficient amount of scattered light does not enter to the image sensor, or no scattered light enters to the image sensor.
Since all wafers do not necessarily have alignment marks of the same shape and the same size, namely, since there is possibility that a number of wafers have alignment marks of different shapes and/or sizes, the conventional mark position determining apparatus has been disadvantageous in that (1) it is in some cases that the position determining operation cannot be performed because a sufficient amount of scattered light is not obtained, (2) a light intensity spectrum of the detected scattered light is vague in some cases, and therefore, the S/N ratio of the image signal obtained is low, so that the side surface detection precision is deteriorated, and a long time is required for the position determining operation, and (3) because the position determining accuracy is deteriorated, the position determining operation often fails and one or more additional position determining operations must be performed.